The problem of accurately drilling holes in large work pieces such as aircraft wing and fuselage panels and other types of structures has been an ongoing challenge in the aircraft industry as well as other industries. A particular challenge is being able to drill holes in a wide range of work piece configurations. Large fixed-mounted machines such as five-axis drilling machines can be used for some types of work pieces, but these machines are quite expensive to procure and operate. In contrast, a relatively low-cost solution to the above-noted problem that has been developed by the assignee of the present application is to mount an automated drill or other machining device on a tract that is mounted to the work piece. The drill or machining device is supported on a carriage that travels along the track, which is formed by a pair of parallel rails mounted on the work piece. For examples of such devices, see U.S. Pat. No. 4,850,763, assigned to the assignee of the present application, and incorporated herein by reference, and U.S. Pat. No. 3,575,364, and U.S. Pat. No. 6,843,328 B2 also incorporated by reference herein.
With many prior devices, such devices were applied to work pieces that did not have compound-contoured surfaces. As used herein, the term “compound-contoured” (also known as “doubly curved”) is used to denote a surface having curvature in more than one direction. On such a compound-contoured surface, it is possible in general to lay a pair of straight, flexible rails such that the rails conform to the surface contour and are the same distance apart at all points along the rails. Thus, the surface of a sphere is an example of a compound-contoured surface, because the rails can be laid in circumferential, axial, or helical directions and the spacing between them can be constant.
With other previously developed devices, a pair of flexible rails is mounted in the circumferential direction around a circular cylindrical work piece. It will be appreciated that the rails were made flexible so that they can conform to a variety of surfaces, but even such flexible rails cannot be positioned exactly the same distance apart at all points along the rails when they are mounted on a compound-contoured surface. Furthermore, the rails mounted along two different paths on a compound-contoured surface will twist differently from one another because of the different directions of the surface normal along the two paths. This can make it difficult to traverse a carriage along the rails and maintain acceptable accuracy of carriage positioning.
With some previously developed devices, a pair of spaced rails is mounted on a compound-contoured surface such that the rails are the same distance apart at all points along the rails. A Y-axis motor and an X-axis motor are supported on a carriage unit that is free to move along the rails. Additionally, the rails are mounted to the surface via vacuum cups. One drawback with such systems is that the rails and the carriage system may not provide enough stability during drilling in order to drill precise holes in a work piece. Moreover, the cost of fabricating drill plates is very costly for this type of two-rail drilling system because a drill plate is made for one specific area of the work piece. Additionally, each drill plate must be made to match the mode line contour of the work piece and establish an exact hole location. Therefore, it is desirable to have a rail system with fewer parts so that an operator can even more easily set up the drilling rail system for use. Additionally, a drilling rail system is needed which provides even better adhesion over seams and holes located on the surface of the work piece, to enable even more accurate and precise holes to be formed in the work piece.